Conventionally, CRTs and liquid crystal displays have been used as display terminals for images such as characters, static images, and moving images. These apparatuses can instantaneously display digital data and rewrite the data. However, these apparatuses have various drawbacks. For example, it is a hard work to always carry such apparatuses. In addition, seeing such a display apparatus for a long period time causes eye fatigue. Further, when power of such display apparatus is turned off, the images are not displayed.
When the characters or static images are distributed or stored as a literature, they are typically recorded by a printer on a paper medium. This paper medium has been used widely as a so-called hard copy. This hard copy is easier to read than those in display apparatuses, the degree of eye fatigue is much less and it can be read by free posture.
Further, hard copies have a characteristic which can be freely carried because of lightweight. However, hardcopies have a problem in view of material saving such that it requires much labor and costs when disposed and recycled after they are used.
Therefore, a need for a paper-like rewritable display medium has been increased which can have both advantages of display apparatuses and hard copies. Until now, display media using a liquid crystal dispersed in a polymer, a bistable cholesteric liquid crystal, an electrochromic element and an electrophoretic element, etc. have been proposed, which are a reflection type and can display high contrast images in addition to memorablility as a display medium.
Among them, apparatuses using an electrophoretic element have good display qualities and low power consumption while display operation (See Patent literatures 1 and 2). In such electrophoretic display medium, a dispersion liquid dispersed a plurality of electrophoretic particles having a color in which color is different from a color of a dispersion medium of the colored dispersion medium is filled between a pair of transparent electrodes. The electrophoretic particles (hereinafter, may be called “migration particle”) have electric charge on a surface of the dispersion medium.
When a voltage having a polarity opposite to the charge of the electrophoretic particles is applied to one of the transparent electrodes, a color of the particles is observed by deposition of the particles on the transparent electrodes. To the contrary, when a voltage having the same polarity as the charge of the electropheretic particles is applied, the particles move toward the transparent electrode of the opposite side, so that the color of the dispersing medium is observed, thereby the images can be displayed.
Therefore, for the electrophoretic particles are required to have an optical property such as white or colored with respect to display state or non-display state, and a good electrostatic property for quick response to move to an outside electric field. Among the particles which have a white color showing a whiteness such as a paper, an inorganic pigment such as a titanium oxide which has a high light-scattering ability has been used conventionally. These inorganic pigments have been used as they are or by adding an additive such as a particle dispersing agent to improve dispersion stability. However, these pigments are usually not complete insulators and a volume resistivity is smaller than the insulator. Thus, in the outside electric filed region in the electrophoretic display, a good migration is difficult along in a direction of the electric field. Particularly, stability for repeatedly switching the display is inferior. Further, there is a problem of display memory in that the display state is changing with time because a specific gravity is remarkably big compared to the dispersion medium, precipitation cannot be prevented. While, in view of insulation and specific gravity as for charging particles, various types of polymer particles are preferable. Many of them migrate very well with respect to the outside electric field. However, among conventional polymer particles, white colored particles having a satisfactory optical property as display materials were not seen.
To satisfy the above two demands, Patent literature 3 has proposed to use a pigment and a polymer being complexed by heat fusion and finely-grinding. However, in this case, a specific gravity between the particle and the dispersion medium are not coincident completely and further there is a limitation in finely-grinding by grinding. The larger the particle diameter, the slower the migration speed is in the electrophoretic display medium, thereby there is a problem in display switching speed. Further, the pigment is not completely contained in the polymer, it may exist on the surface in a certain degree. Therefore, there occurs non-uniformity in charging the particles.
The above problem is not fully resolved at the present stage.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 05-173194    Patent Literature 2: Japanese Patent (JP-B) No. 2612472    Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 48-31097